[M68k] selection failure on load from global

This seems closely related to https://github.com/llvm/llvm-project/issues/200826 (closed by https://github.com/llvm/llvm-project/pull/201653). I hit this when compiling the rust standard library.

https://godbolt.org/z/jaehYq6oG

```llvm
@counter = internal global i64 0, align 8

define void @foo(i32 %x) {
entry:
%p = getelementptr inbounds i8, ptr @counter, i32 4
store i32 %x, ptr %p, align 4
ret void
}
```

For `-mtriple=m68k-unknown-linux-gnu` gives

```
LLVM ERROR: Cannot select: t8: ch = store<(store (s32) into %ir.p)> t0, t3, t6, poison:i32
t3: i32,ch = load<(load (s32) from %fixed-stack.0, align 8)> t0, FrameIndex:i32<-1>, poison:i32
t6: i32 = add nuw t12, Constant:i32<4>
t12: i32 = M68kISD::WrapperPC TargetGlobalAddress:i32 0
In function: foo
```

Reproduce the failure with the LLVM IR snippet and -mtriple=m68k-unknown-linux-gnu, then trace the M68k instruction-selection path around M68kISD::WrapperPC and the global address. Done means the example compiles without the reported LLVM ERROR, including the Rust standard-library case that exposed it.

Written by the indexing model from the issue text.